This is our second year in this house. Our plum tree comes on beautifully with foliage and flowers nicely. Shortly after the blooms are gone the lower half of the tree begins to dry up. The top half continues to grow nicely and produces a good fruit. Unfortunately when we pruned the tree this year we forgot that the lower half 'dies'. Now there is very little green at the top and I doubt if any fruit will be there, but the tree is still alive. What could be causing the lower have to die back to twigs? What can be done for it?